Both Notes Formula Language and LotusScript allow you add a field to a data document on the fly, whether that field exists on the Form associated with that document or not.
Example: FIELD DateTime : @Now; document.dateTime = Now
Even if the field doesn't exist in the form used to view the document, it's still there. It can still be displayed in views, exported to other formats (via the gui) and referenced and manipulated by your code.
Why is it important to add the field to the form? Because, if you ever end up accessing that data using an external tool, such as LEI, or Notes ODBC, and you want to reference the data you are after by Form, you will miss any data that is stored in fields that are not on that form. An example of this is if you archive a database using LEI (since built-in archiving won't write to a different server on a schedule). If you archive documents based on Form, none of the data that was added on the fly will be present in the documents in the archive. And that could be bad.